1. Field of the Invention
The present invention generally relates to stereotactic mammographic guided needle breast biopsies, and more particularly to an add-on stereotactic needle breast biopsy apparatus for converting a conventional mammography apparatus into a mammography apparatus capable of taking high quality stereotactic mammographic images of a breast and performing stereotactically guided needle breast biopsies on lesions identified in the stereotactic images.
2. Description of the Prior Art
Within the last couple of decades there have been many improvements made in the field of mammography and stereotactic mammographic guided needle breast biopsies. These improvements have allowed the field to move towards faster, more accurate and less invasive procedural techniques to determine whether a suspicious lesion spotted in a mammographic image is malignant. More particularly, the field of mammography, as with many other medical imaging procedures, is shifting from traditional screen-film based imaging, where mammographic images are captured on a film negative, to digital imaging, where such images are acquired on a Charge Coupled Device (CCD) array and displayed on a cathode ray tube (CRT). While the advantages of the latter method for imaging are numerous, one of the most readily notable advantages is the significant reduction in image acquisition time. For example, an image captured on a CCD array may be displayed on a CRT in as little as three to six seconds after the mammographic imaging procedure is completed. In contrast, it can take more than 5 to 10 minutes to obtain the same image captured on a film negative due to processing time requirements involved in developing the mammogram film negative. The reduction in image acquisition time has been a real benefit to patients undergoing a stereotactically guided needle breast biopsy. An example of a digital imaging camera system employing a CCD array which is suitable for use in mammography may be found in the applicants' U.S. Pat. No. 5,216,250, which is incorporated herein by reference in its entirety.
Stereotactic mammographic guided needle breast biopsy procedures are evolving towards sampling cells of suspicious lesions through less invasive fine needle aspiration (FNA) procedures and sampling tissue of suspicious lesions through needle core biopsies, and away from using more invasive procedures, such as wire guided surgical excision of the suspicious lesion. Generally, there are two types of stereotactic mammographic guided needle breast biopsy devices, add-on and dedicated, described in the art for performing the range of stereotactically mammographic guided needle breast biopsies described above.
A prior art add-on biopsy device allowing stereotactic mammographic breast biopsy procedures to be carded out using a conventional mammography apparatus is disclosed in U.S. Pat. No. 4,727,565. The add-on biopsy device described therein comprises a needle guiding stage, a compression paddle and an image receiver. When a stereotactic guided needle biopsy is desired, the biopsy device is attached to a conventional mammography device. The patient's breast is held in a compressed state between the compression paddle and image receiver. To acquire the stereotactic images, the device employs oblique angle stereotactic imaging geometry wherein the X-ray tube of the mammography apparatus is positioned at oblique angles relative to the plane defined by the image receiver. Once the stereotactic images are obtained, the two dimensional positional coordinates of the suspicion lesion appearing in each of the images is measured and these two dimensional positional coordinates are used to calculate the three dimensional coordinates of the suspicious lesion in the breast relative to the needle guiding stage. Unfortunately, the oblique angle stereotactic imaging geometry employed by this device to acquire stereotactic images has some drawbacks which can possibly compromise the image quality of the stereotactic images. The imaging geometry drawbacks will be more fully explained below.
A prior art dedicated biopsy device for carrying out stereotactic mammographic breast biopsy procedures is described in "Stereotaxic Instruments for Needle Biopsy of the Mamma", an article authored by Jan Bolmgren et al, published in the American Journal of Roentgenology, Vol. 129, page 121, in July 1977. In the dedicated biopsy device, the patient is positioned on a table in a prone position over the imaging and biopsy apparatus. The breast is pendulantly presented through an aperture at one end of the table. The obvious advantage of the dedicated biopsy device over the add-on biopsy device is that it is far more comfortable for the patient. However, the dedicated biopsy device tends to be relatively more expensive and tends to take up more floor space, which, in some situations, can also be expensive. Typically, the dedicated device is only used for stereotactic mammographic breast biopsy procedures so it may have somewhat less overall utility than a conventional mammography device equipped with an add-on stereotactic biopsy apparatus. A commercial version of this device, known as a TRC Mammotest was manufactured by Tekniska Roontgencentralen AB of Sweden. A description of the commercial TRC Mammotest may be found in U.S. Pat. No. 5,078,142. The two dedicated devices identified herein also suffer from the same oblique angle stereotactic imaging geometry drawbacks as mentioned above for the prior art add-on device.
While there is little doubt that oblique angle stereotactic imaging geometry, briefly described in the '565 patent, is sufficient for calculating the three dimensional coordinates of an observed lesion in a pair of images, it is believed that this stereotactic imaging geometry is not the best for obtaining stereotactic images with optimum image quality, quality approaching that of conventional screening mammography. More particularly, because these devices position the X-ray tube at an oblique angle relative to the plane of the image receiver for each of the stereotactic images, it is nearly impossible to use a conventional moving scatter reducing grid in a conventional manner because the central ray of the X-ray beam does not fall normal to the plane of the image receiver.
Conventional scatter reducing grids generally comprise a plurality of nearly parallel slats of X-ray absorbing materials that are typically positioned to be focused at the focal spot of the X-ray tube. These grids are also moved in a direction tangential to the patient's chest wall during the X-ray exposure to blur the shadows cast on the film by the plurality of X-ray absorbing materials. Because the central ray of the X-ray beam of the above described prior art devices is not presented normal or perpendicular to the plane of the X-ray film in either of the stereotactic imaging positions, it is nearly impossible to use a moving scatter reducing grid in the conventional manner because the oblique angle stereotactic X-ray imaging positions take the focal point of the X-rays outside of the focus of the grid. While it is possible that one skilled in the art could orient a conventional grid such that it could be used during stereotactic imaging with these prior art devices to somewhat overcome the above described constraints, these prior art biopsy devices have a further drawback in that they cannot use scatter reducing grids having X-ray absorbing materials arranged in a crossed structure because there is no way to orient the grid in the image plane such that this type of grid will be in focus with the focal spot of the X-ray tube during stereotactic imaging.
Thus, it is possible that a suspicious lesion spotted in a mammogram obtained by standard screening mammography techniques using a conventional scatter reducing grid may not be observable in stereotactic images taken with these prior art devices because they are generally unable to effectively use the scatter reducing grids. A brief discussion of the benefits of scatter reducing grids may be found in the Recommended Specifications for New Mammography Equipment published by the American College of Radiology.
The above noted limitations of the oblique angle imaging geometry of the prior art add-on and dedicated devices have been overcome by the stereotactic mammography imaging system described in related grandparent U.S. Pat. No. 5,289,520, which is incorporated herein by reference in its entirety. The device disclosed therein is a dedicated stereotactic mammography biopsy system. It overcomes the oblique angle imaging limitations by obtaining stereotactic images of the breast with perpendicular stereotactic imaging geometry wherein the focal spot of the X-ray tube is always presented normal to the plane of the X-ray film in each of the stereotactic imaging positions. Unlike the oblique angle stereotactic imaging geometry of the prior art devices, the perpendicular stereotactic imaging geometry does not place the same constraints on the use of scatter reducing grids since the X-ray focal spot remains in the same position relative to the image receiver during all stereotactic imaging. Thus, a conventional scatter reducing grid will always be in focus with the focal spot of the X-ray tube during all stereotactic imaging. However, while this device is more comfortable for the patient, as explained above, dedicated biopsy devices may have somewhat less overall utility for the user and can be more expensive than a typical mammography-apparatus equipped with an add-on type stereotactic needle biopsy device.
For at least several years, some mammography apparatus have been equipped with digital image receivers having a CCD camera focused through an optical arrangement of mirrors and lens or optical fibers at a phosphor screen. As compared to film-screen image receivers, digital image receivers tend to have a small field of view due to the costs of CCD arrays. When a mammography apparatus equipped with a digital image receiver having the small field of view is used to acquire stereotactic images and perform needle biopsies, conditions can arise such that the resulting stereotactic X-ray images do not completely fall within the field of view of the CCD camera. Accordingly, it is possible that a lesion in some of the stereotactic X-ray images can fall outside of the field of view in one or even both stereotactic images.